Encyclopedia of Environmental Science and Engineering, Volume I and II

570 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

immobilized Escherichia coli, having glutamate decarboxyl-

ase enzymic activity, participate in the following reaction:

(30)

The sample solution is at pH 4.4, low enough to assure the

presence of CO 2 (pK  6.34) instead of HCO 3  (see equa-

tion 24). This electrode responded very slightly to some

other amino acids.

A linear dynamic range is obtained in the following terms:

a linear relationship exits between the log of the concentra-

tion range, 100–800 mg/L, vs. the potential of the CO 2 elec-

trode. The standard deviation, sd, is 1.2 mg/L. Measurement

time is 5 minutes. The sensor functioned consistently for

more than three weeks and 1,500 determinations.

(y) Amperometric type

An example of a microbial electrode of use in wastewater

analysis is the ammonia electrode illustrated in Figure 24. The

nitrifying reactions are as follows:

NH^32 1.5O Nitrosomonas sp NO^22 H O H

→  (31)

NO^22 0.5O Nitrobacter sp NO^3

 →.

(32)

Oxygen concentration is detected, amperometrically, by the

Pt electrode. The current decrease is linearly related to the

NH 3 concentration when the NH 3 concentration is below

42 mg/L; the minimum detectable NH 3 concentration is 0.1

mg/L. This sensor has about a 4 min. response time.

Karube uses a bacterial electrode employing Trichosporon

cutaneum to measure biochemical oxygen demand in waste-

water treatement. The difference between the steady state

and initial electrode current was correlated to 5-day BOD,

BOD 5.^71 A BOD 5 measuring system^52 utilizing a different

microbial electrode is described in Section 5, b,(3), ( c ),( ii ).

b. Voltammetric and amperometric procedures

Voltammetry and amperometry are used to determine the

concentration of an electroactive analyte through current mea-

surement under various potential regimes. In voltammetric

procedures the current is measured as a function of the poten-

tial scanned between two potential values. Amperometric

methods measure current at a fixed potential.

Amperometric methods apply to electrochemical sen-

sors which are used directly or in a variety of titrimetric

methods. Voltammetry can be divided into two categories:

TABLE 9

Microbial electrodesa

Analyte Ref. Organism Electrodeb

Glucose Pseudomonas fluorescens oxygen, Amp. (59)

Alcohol Trichosporon brassicae oxygen. Amp. (60)

Acetic acid Trichosporon brassicae oxygen, Amp. (61)

Formic acid Clostridium butyricum hydrogen, FC (62)

Glutamic acid Escherichia coli CO 2 , GS (57) (58)

Ammonia Nitrosomonas sp. and Nitrobacter sp. oxygen, Amp. (63) (64, 65)

Methane Methylomonas flagellata oxygen, Amp. (66) (67)

Nitrite Nitrobacter sp. oxygen, Amp. (68) (69)

BOD Clostridium butyricum or Trichosporon

cutaneum

bio, FC (70) (71)

Mutagen B. subtilis, recombination deficient oxygen, Amp. (72) (73) (74)

a From Ref. (52).

b Amp.—amperometric; GS—gas sensing; FC—fuel cell.

1

2

3

4

5

6

7

8 9

FIGURE 24 The microbial sensor system for

ammonia. 1. Electrolyte (NaOH); 2. Cathode

(Pt), 3. Immobilized cells of nitrifying bacteria;

4. Magnetic stirrer; 5. Gas permeable Teflon
   membrane; 6. Teflon membrane; 7. Anode
   (Pb); 8. amplifier; 9. recorder. (Reprinted with
   permission from reference 52. Copyright 1986
   American Chemical Society.)

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